I am almost novice to graphic design. I need to know whether there is any way to create mock-ups in GIMP, people generally create in Photoshop with PSD format. Is there any format like PSD in GIMP, which can be used to create mock-ups easily?
Yes. GIMP's native file format is XCF. It's the format you should use to save your original work for future editability (whether it's being used as a template or not). It's a lossless format, and it preserves all layers, and editable text layers.
Although GIMP also supports PSD to some degree, you should use XCF for full compatibility with GIMP. However, Photoshop doesn't support XCF files at all, so if you need to open it in Photoshop, then you would need to export the image as a PSD from GIMP.
Edit (suggested by Joonas)
Note that GIMP doesn't have any Smart Object functionality at present, so any template/mockup design created with GIMP will necessarily involve more manual work in order to use it, when compared to Photoshop mockups that use Smart Objects to automate placement/transforms of images. There are future plans by GIMP's developers to implement something similar to Smart Object functionality, but it's still some way off, unfortunately.
So are there ways to work around this handicap?
There are certainly possibilities, such as keeping a group with original editable elements hidden, which could then be duplicated, edited, then flattened to create a single layer which could then be transformed to fit on a mock-up. Obviously, this will still involve manual operations, and it's less than ideal and not as convenient as Smart Objects, but it could still work.